This invention relates to the field of automatic positioning chairs for medical application. Such chairs are sometimes referred to by the general term "table" and when the word "chair" is used herein, it is intended to include such a "table". The invention has particular application to podiatry chairs which may be automatically elevated or tilted and which may have automatically moveable back supports or foot supports.
Prior art chairs of the type with which this invention is concerned use feedback sensors for measuring the position of moveable surfaces and signaling the measured position to a motor controller. Automatic positioning control is achieved by comparing a position command with a measured position to develop an error signal. The position motors are driven until the error signal has been reduced to zero. At this point, the controller knows that the desired position is achieved.
Prior art chairs of the above described type have also been provided with control units having switches which may be operated to move various support surfaces to desired locations. When the desired locations are achieved, the operator activates a switch directing the control system to store the positions of all moveable surfaces. Thereafter the chair may be returned to the same positional configuration by operating the same or another switch.
There has been a need for automatically positionable chairs capable of returning to a programmed position without the use of feedback sensors. Such sensors complicate the design and increase the cost of the chair. It is apparent that such a chair may be operated "open loop" without position sensors by simply measuring the time required to move the chair surfaces to a desired position and treating such movement time as a measurement of position. However, it is desired to move such chairs to a variety of preprogrammed positions under various load conditions which may be encountered during movement with patients of differing weights seated thereon. Drive motors are prone to operate at different speeds under different load conditions. This is especially true for electric motors which are designed with the anticipated operating load in mind and which do not have substantial overcapacity. It is impractical to utilize elapsed movement time as a measure of surface position when the motor speed changes with load.